In a packet-based digital communications system, the properties of a transmission channel must be accurately estimated in order to reliably recover the information in each received packet. In the case of a multiple access network, a packet can be transmitted by any one of several transmitters, each of which is in a different location. This results in different transmission channels from each transmitter. In a mobile wireless network, a single transmitter may change locations between two successive transmissions, resulting in different channel transmission characteristics. In all of these cases, the characteristics of the transmission channel over which the packets travel needs to be estimated to accurately set the receiver parameters and recover the transmitted data. In addition, the receiver parameters need to be periodically adjusted in order to adapt to the changes in the channel characteristics.
In a packet-based communications system, packets are typically composed of message data and a preamble. The preamble contains a data sequence which allows the receiver to estimate the channel parameters.
To adaptively estimate the channel parameters using known techniques such as the Least Mean Squares (LMS) or Recursive Least Squares (RLS), a decision signal and a decision error signal are used to adjust the coefficients of an adaptive equalizer. Such equalizers are well known in the art. The decision signal and decision error signal are obtained either from a training sequence known by both the transmitter and receiver, or by use of a decision-directed method where the receiver uses an estimate of the decision and decision error signals to adjust the receiver parameters.
A known training sequence is frequently used to adapt the equalizer parameters. Since the training sequence is known, the decision is made with absolute certainty and the decision error can be used to precisely determine channel impairments. One drawback with this approach is that the training sequence must be synchronized with the incoming received signal.
In decision-directed adaptation, the decision is made by a hard decision slicer and the system does not utilize the benefit of the coding gain achieved through error control. Decision-directed adaptation can be useful for continually adapting a received signal, but may be sub-optimal with respect to initially converging the adaptive equalizer.
For the foregoing reasons, there is a need for a reliable and efficient method for converging and adjusting adaptive parameters in a communications receiver which simultaneously allows the system to benefit from the coding gain achieved through error control.
The present invention encompasses a packet based communications receiver which utilizes the overhead portion of a received data packet to converge an adaptive equalizer, and then applies decision-directed adaptation to the received message symbols.
In a preferred embodiment, the overhead symbols are produced using a reliable modulation code, and a modulation decoder is used in combination with a data re-encoder to generate an estimate of the overhead symbol. An equalizer is used to simultaneously generate an equalized version of the received overhead symbol, and the difference between the equalized version and the estimate is used to adjust and converge the coefficients in the adaptive equalizer.
One advantage of the present invention is that reliable or very robust modulation codes can be used in the overhead portion of the data packet, and provide a high degree of certainty that the overhead symbols will be received correctly, even in the absence of equalization. The message portion of the packet can be encoded using less redundant (sparse) codes, resulting in a higher data throughput. The message portion can be received and passed through the adaptive equalizer.
The system can utilize a number of reliable modulation codes including Barker codes, repetition codes, or modulation codes. In a preferred embodiment a rate 1/11 Barker code is utilized.
The system can converge the adaptive equalizer by decoding received overhead symbols to produce an estimate of the overhead data, re-encoding the estimate of the overhead data to generate an estimate of the overhead symbols, and comparing this estimate of the overhead symbols with an equalized version of the overhead symbols. The result of this comparison is a decision error signal which is used to optimize the tap coefficients of the adaptive equalizers. A number of optimization algorithms can be used in the adaptive equalizer and include the Least Mean Squares (LMS) algorithm as well as the Recursive Least Squares (RLS) algorithm.
In a preferred embodiment, delay is introduced in the adaptive equalizer path such that the total delay through the modulation decoder and the data re-encoder is approximately equal to the delay through the delay element and the adaptive equalizer.
In a preferred embodiment, the system comprises a modulation decoder unit which provides modulation decoding of a received digital signal. A data re-encoding unit is coupled to the modulation decoder for generating an estimated signal. An adaptive equalizer unit is provided for equalizing the received digital signal and for producing an equalized signal. A difference unit monitors the difference between the equalized signal and a reference signal which can comprise, for example, the estimated signal. The output of the difference unit is coupled to the adaptive equalizer.
In an alternate embodiment, the system comprises a modulation decoder unit which provides modulation decoding of a received digital signal. A data re-encoding unit is coupled to the modulation decoder for generating an estimated signal. An adaptive equalizer unit is provided for equalizing the received digital signal and for producing an equalized signal. A difference unit monitors the difference between a reference signal (e.g., the estimated signal) and the equalized signal, with the output of the difference unit being coupled to the adaptive equalizer. A slicer generates an output signal containing decisions from the equalized signal. A switch is coupled to the difference unit. During a training period corresponding to reception of the overhead portion, the switch routes the estimated signal to the difference unit. During normal operation corresponding to reception of the message portion, the switch routes the output signal from the slicer to the difference unit.
One advantage of the present invention is that it can be applied to continuous communications systems as well as to packet based systems. In applying the invention to a continuous system, the adaptive equalizer can be periodically trained using reliable modulation codes.
The present invention allows for the rapid and accurate training of an adaptive equalizer, and can result in the reliable reception of data in a number of communications systems including wireless systems. Given the difficult transmission characteristics of many wireless systems, which are subject to multipath fading, Rayleigh fading, and other transmission impairments, the present invention allows an adaptive equalizer to be rapidly trained to accurately recover the data from the received signal.
These and other features and objects of the invention will be more fully understood from the following detailed description of the preferred embodiments which should be read in light of the accompanying drawings.